The present invention relates to a light emitting diode (LED) used as a general-purpose light source, as a back lighter for the pushbuttons of a mobile phone, or as a light emitting element of a photointerrupter for example. The present invention also relates to an illuminator using such an LED.
As shown in FIG. 5, a typical LED basically includes a chip body 10, a lower electrode 4 formed entirely over a lower surface of the chip body 10 and an upper electrode 105 formed on an upper surface of the chip body 10. The chip body 10 includes an n-type (or p-type) semiconductor layer 1, a p-type (or n-type) semiconductor layer 2, and a light emitting layer (active layer) 3 interposed therebetween. The chip body 10 is rectangular as viewed in plan and has a predetermined thickness. When energized, light emitted from the light emitting layer 3 travels outward through the upper surface and the side surfaces of the chip body 10. The lower electrode 4 is made of Au which has a good conductivity and is formed over the entire lower surface of the chip body 10. Similarly to the lower electrode 4, the upper electrode 105 is made of Au and generally has a thickness of about 1 xcexcm. The upper electrode 105 covers only a central portion of the upper surface of the chip body 10 so that light can be emitted upward through the upper surface of the chip body 10.
Generally, the LED has a laminated structure of gallium-arsenic compound layers such as GaAlAs layer. Specifically, the LED is formed by preparing a semiconductor wafer by successively forming, on a substrate, a p-type GaAlAs semiconductor layer, a GaAlAs light emitting layer and an n-type GaAlAs semiconductor layer by epitaxial growth, forming an lower electrode 4 and an upper electrode 105 on the semiconductor wafer, and dividing the wafer into chips by dicing. At this time, the upper electrode 105 is provided by forming an Au film entirely over a surface of the semiconductor wafer and then etching the Au film into a predetermined configuration (typically circular).
FIG. 6 illustrates the structure of a typical illuminator using an LED. The LED illuminator may be used as a light source for example. The LED illuminator includes a first lead T1 connected to the lower electrode 4 of the LED, a second lead T2 connected to the upper electrode 105 of the LED via a wire W, and a light-permeable resin package P. The resin package P entirely seals the LED and the wire W while partially sealing the first lead T1 and the second lead T2.
The resin package P may often formed of a light-permeable epoxy resin which does not contain a filler, because it is relatively inexpensive and can be easily hardened by heating to provide the package. The resin package P is formed by setting the LED, the wire W, the first lead T1 and the second lead T2 in a cavity of a predetermined configuration defined by a mold, injecting the melted epoxy resin into the cavity, and heating the resin for hardening.
In using the LED illuminator, power is applied across the first lead T1 and the second lead T2 to turn on the LED. At this time, the LED is heated due to the light emission, thereby heating the resin package P therearound. Since the light-permeable epoxy rein which does not contain a filler has a considerably large coefficient of linear expansion, the resin package P thermally expands due to the heat.
As described above, in the LED, the upper electrode 105 covers only the central portion of the upper surface of the chip body 10. Thus, the upper surface of the chip body 10 is partially exposed for contact with the resin package P. Moreover, the gallium-arsenic compounds forming the chip body 10 are more fragile than silicon which is used for forming general semiconductor chips.
Therefore, the LED may break when the exposed portion of the upper surface of the chip body 10 is pressed by the package P. Particularly, the p-type (or n-type) semiconductor layer 2 has edge portions 2a which suffer strains due to the stresses generated in cutting the wafer and hence are likely to break. As the breakage of the chip body 10 progresses during the use, the light-emittable region gradually becomes smaller, thereby deteriorating the brightness of the LED.
Such a breakage of the chip body 10 may also occur in forming the resin package P. Specifically, when a melted epoxy resin is injected in a cavity and heated for hardening, the epoxy resin thermally expands in the cavity to press the LED 100. As a result, the chip body 10 may break from the edge portions 2a of the p-type (or n-type) semiconductor 2.
The breakage of the chip body 10 may be prevented by subjecting the LED to mesa treatment for solubly removing the edge portions 2a, 2b, or by covering the LED with a cushion C within the resin package P as shown in FIG. 7. However, such a method makes the manufacturing process complicated, thereby deteriorating the manufacturing efficiency of the LED and the LED illuminator.
Moreover, although the light emitting layer 3 is formed over the entire horizontal section of the chip body 10, the upper electrode 105 covers the upper surface of the chip body 10 only partially. Therefore, the current density relative to the planar area of the chip body 10 is low, which leads to a low luminous efficiency of the LED as a whole. Therefore, to obtain a desired amount of light, the LED or the LED illuminator cannot be decreased in size.
An object of the present invention is to provide an LED which is capable of preventing the brightness from deteriorating without lowering the manufacturing efficiency and which can be made compact.
Another object of the present invention is to provide an illuminator utilizing such an LED.
According to a first aspect of the present invention, there is provided a light emitting diode comprising a chip body including an n-type semiconductor layer, a p-type semiconductor layer and a light emitting layer interposed therebetween, a lower electrode formed on a lower surface of the chip body, and an upper electrode formed on an upper surface of the chip body. The upper electrode is formed entirely over the upper surface of the chip body.
Preferably, the upper electrode is formed of gold.
In the LED having the above-described structure, the upper electrode is formed entirely over the upper surface of the chip body so that the current density relative to the planar area of the chip body is high. Therefore, although the light is emitted only through the side surfaces of the chip body, the light emitting efficiency of the LED as a whole can be enhanced. As a result, the LED and the illuminator using the LED can be made compact.
Moreover, in the LED, the upper surface (particularly at the edge portions) of the chip body, which is likely to break, is covered with the upper electrode for protection. Therefore, unlike the prior art, the breakage of the LED in use can be prevented without chamfering the edge portions of the upper surface of the chip body by mesa treatment or without forming a cushion for protecting the LED in making an LED illuminator. Therefore, it is possible to prevent a reduction of the light-emittable region. As a result, the degradation of the brightness can be prevented without lowering the manufacturing efficiency of the LED and the LED illuminator.
According to a second aspect of the present invention, there is provided an illuminator comprising a first terminal, a second terminal, and a light emitting diode mounted on the first terminal and electrically connected to the second terminal. The light emitting diode comprises a chip body including an n-type semiconductor layer, a p-type semiconductor layer and a light emitting layer interposed therebetween, a lower electrode formed on a lower surface of the chip body and an upper electrode formed on an upper surface of the chip body. The upper electrode is formed entirely over the upper surface of the chip body.
Preferably, the upper electrode is formed of gold, and the upper electrode is connected to the second terminal via a wire.
The illuminator may further comprise a light-permeable resin package for sealing the light emitting diode and the wire and part of the first and the second terminals.
Preferably, the first terminal has an upper end formed with a recess for accommodating the light emitting diode. The recess flares upward.
Other features and advantages of the present invention will become clearer from the description of the embodiment given below with reference to the accompanying drawings.